FIG. 1 shows in simplified block diagram a "Macintosh SE/30" personal computer, generally designated by the reference numeral 10. The "SE/30" is manufactured by Apple Computer, Inc., of Cupertino, Calif. It has a box-like computer housing (not shown) which generally contains everything necessary for computer operation except a keyboard and a printer. A microprocessor motherboard 12 integrates most of the necessary electronics, including floppy and hard disk drive interfaces, communications ports, and video circuits. Motherboard 12 also provides a single expansion slot 13.
The "SE/30" has an integrated, internal CRT (cathode ray tube) 14 mounted within the computer housing. CRT 14 is driven by the motherboard video circuits and additionally by a sweep board 16 and an internal CRT amplifier 18, both of which are separate from motherboard 12 but still contained within the computer housing.
Integrated on motherboard 12 is a two-state video driver 15 which generates a binary video signal BVID, a vertical synchronization signal VSYNC, and a horizontal synchronization signal HSYNC. The motherboard video signals are all binary, logic-level signals. Therefore, at any time each motherboard video signal has either a "high" voltage, typically approximately five volts, or a "low" voltage, typically approximately zero volts.
Sweep board 16 receives the motherboard video signals. It translates the logic-level VSYNC and HSYNC signals to the voltage levels required by CRT 14 and provides them as VSWEEP and HSWEEP to CRT 14. Sweep board 16 reduces the amplitude of motherboard binary video signal BVID from five volts to one volt, producing a reduced amplitude binary video signal RVID. Sweep board 16 also performs other functions related to driving CRT 14, such as producing grid and heater voltages.
CRT amplifier 18 normally receives the reduced amplitude binary video signal RVID, producing in response an amplified video signal AVID to drive the cathode of internal CRT 14. Amplified video signal AVID has a peak voltage of about thirty volts.
Because the video circuits of the "SE/30" computer are primarily integrated on its motherboard, rather than on a removable expansion board, upgrading to a multi-shade or color video display system is essentially prevented.
This invention resolves the need for an acceptable way of upgrading the "Macintosh SE/30" computer and similar computers to display multi-shade images, providing as well further new and innovative features which allow the "SE/30" computer to be used in modern and more complex applications than had previously been possible.